In recent years, an automotive internal combustion engine, even in a rotary type, with more than three intake ports for each cylinder has been developed to improve charging efficiency of intake air so as to increase engine horsepower. An intake system, used for such multi-port automotive internal combustion engine as having three intake ports, namely primary, secondary and auxiliary supplementary intake ports, is adapted to carry intake air to the cylinder through the primary intake port in low engine load conditions, the primary and secondary intake ports in middle engine load conditions or all the three intake ports in high engine load conditions. Examples of such a rotary internal combustion engine and intake system is disclosed in, for example, Japanese Unexamined Patent Publication Nos. 60(1985)-93124 and 61(1986)-251422, respectively.
The three port internal combustion engine is generally designed to close the primary and secondary intake ports in close proximity and the auxiliary intake port with a delay of significant time behind the primary and secondary intake ports. This is because, kinetic effects, such as inertia effects or resonance effects, of intake air generally decrease charging efficiency of intake air at high engine speeds and such a decrease of charging efficiency must be avoided.
An intake system conventionally used in cooperation with the three port internal combustion engine has first and second discrete intake passages branching off from, for example, a surge tank which is connected to the primary and secondary intake ports of each cylinder, respectively for the reason of providing the individual discrete intake passage having a length suitable for inertial supercharging, of giving the individual discrete intake passages a sufficient volume to prevent a delay of intake on acceleration and of causing the engine to change smoothly its output power according to changes of engine load at the commencement or the end of intake air through the secondary intake port. The auxiliary intake port is connected to either the first discrete intake passage communicating the cylinder through the primary intake port or the second discrete intake passage communicating the cylinder through the secondary intake port in an attempt at avoiding the complex and bulky structure of the intake system.
Because the first and second discrete intake passages are formed by relatively large pipes, the intake system is, however, not immune to structural complexity and bulkiness. Furthermore, although the primary or the secondary intake port itself enjoys an inertial charging effect by providing an appropriate length of discrete intake passage therefore, from the viewpoint of the intake system as a whole, inertial charging effect is considerably decreased due to such an interference of compressed pressure waves between the primary and secondary intake ports that, for example, compressed pressure waves introduced into a combustion chamber of the cylinder through the secondary intake port immediately before the secondary intake port has been closed blows out through the primary intake port, or vice versa. That is because, although the primary and secondary intake ports are in close proximity in close timing, compressed and expanded pressure waves, which are out of phase, are introduced into the discrete intake passages, respectively.
Because of the discrete passage for the auxiliary intake port common to either the first or the second discrete intake passage, it is impossible to provide an optimum length of passage which can introduce compressed pressure waves immediately before the auxiliary intake port is timely closed, so that intake air is difficult to be introduced through the auxiliary intake port with sufficiently enhanced inertial charging effects.